Synthesis and characterization of mesoporous lithium metal phosphates (LiMPO4) (M= Mn, Fe, Co, Ni)

Abstract

Synthesis of mesoporous lithium metal phosphates have been studied extensively in past after the emerge of lithium iron phosphate as a cathode material in the lithium ion batteries. These materials are proved to be modifiable and useful in lithium ion batteries. This study encompasses synthesis and characterization of the mesoporous LiMPO4 (M= Mn(II), Fe(II), Co(II), and Ni(II)) from lyotropic liquid crystalline (LLC) mesophases, utilizing a method which can be described as a modified molten salt assisted self-assembly (MASA) method. Preparation of clear solutions and LLC mesophases afterwards are quite an effortless process, once optimized, which in its order starts with the clear solution prepared for the synthesis of lithium transition metal phosphate, then the coating of the solution over glass substrate using two methods, the spin coating and drop-casting. The coated films are then calcined to fabricate the mesoporous lithium metal phosphate products. In this thesis, the mesoporous LiMPO4 (M = Mn(II), Fe(II), Co(II), and Ni(II)), are synthesised using the modified MASA method using 10-lauryl ether as the soft template and characterized using multi-analytical techniques (such as FTIR, PXRD, SEM, EDX, and N2 adsorption-desorption). In the initial part of the thesis, the solution stability over time, pH dependence, and concentration of the ingredients were investigated. It was found that through time these solutions precipitate ranging from weeks to hours with an inverse relation with the concentration of used salts, and acid relative to the surfactant. Continued in this part, it was observed that solution stability is also dependent on pH, which was tested using LiOH instead of LiNO3 as the lithium source. It was revealed that, at higher pH values, the solutions are less stable and produce more precipitate. The solutions, prepared using Mn(II), Fe(II), Co(II), and Ni(II), were coated on glass substrates by drop-cast coating and spin coating methods. These two methods were used to determine the best method for a desired amount and morphology of the corresponding products. After testing a broad range of ingredient concentrations, using the Mn(II) system, three concentrations were selected to represent dilute, medium and concentrated ratios of salt and acid versus the surfactant. The aging and temperature dependent changes were monitored using FT-IR spectroscopy; the effect of temperature on both the formation of mesophase and the reactions taking place in the mesophase has been investigated. It appears that the temperature has some profound effects on the mesophase. The mesophase gets disordered by increasing temperature. This trend also correlates well with increasing salt concentration in the media. As the salt concentration increases the temperature required to disrupt the mesophase decreases. The FT-IR spectroscopy study shows that; significant amount of nitrate species and surfactant molecules have been removed from the media at around 160oC. To remove the surfactant completely, minimum temperature of calcination determined to be 250oC. Samples, prepared with low concentration solution of Mn(II) salt coated with both methods, were calcined at 250, 350, and 450oC and characterized using XRD, FT-IR spectroscopy and SEM techniques. It was found that the drop-casting method is favourable over the spin coating method, because the spin coating method failed to produce the desired compound and created metal pyrophosphate instead of lithium metal phosphates. LiMnPO4, LiFePO4, LiCoPO4, and LiNiPO4 were synthesised using drop-cast coating method and characterized by XRD, FT-IR spectroscopy, SEM, EDX, and N2- adsorption-desorption techniques. It was found that these materials are mesoporous and have noticeable surface areas with some by-products. The pores are large and nonuniform in LiMnPO4 and LiCoPO4, but the pores are small (3-6 nm range) in the iron and nickel samples. The surface area also accords with observation and highest (96 m2/g) surface area was recorded from nickel samples. The pores gradually expand with annealing the samples and becomes non-uniform all cases. The undefined crystalline phases require more work to determine their structure and more optimization to obtain the desired material.